Ah. I mis-interpreted what you meant by "market value change". So, yes, your formula is the same as my SWAG. You adjust both start & end values by 1/2 of the total contribution, same as my SWAG.

I used to use XIRR in Excel for my portfolio computations many years ago, but don't anymore since I discovered that this SWAG formula gives almost the same result for regular periodic contributions.

BTW, you don't include dividends in the XIRR, just additions & withdrawals.

It becomes quite complicated when you have irregular contributions and/or a mix of both additions & withdrawals. Like when you want to figure your returns for a taxable account that you shift money into and out of. But XIRR takes all that in stride, whereas none of the simpler methods do.

When Life Gives You Lemons
We all have had hardships and made poor decisions. The important thing is how we respond and grow. Read the story of a Fool who started from nothing, and looks to gain everything.

Work for Fools?
Winner of the Washingtonian great places to work, and Glassdoor #1 Company to Work For 2015! Have access to all of TMF's online and email products for FREE, and be paid for your contributions to TMF! Click the link and start your Fool career.